Feedback control is an important technique for obtaining high quality servo responses. To monitor system operation, sensors are used to observe the desired signals, which could be temperature, torque, speed, current, voltage, etc. An ideal sensor, accompanied with a related signal conditioning circuit, should have the following characteristics:
(a) cost-effective; PA1 (b) compact size and flexible packaging capability; PA1 (c) accuracy over a temperature range from -40.degree. to +125.degree. C. with high bandwidth; PA1 (d) high common-mode rejection capability; PA1 (e) a ground referenced output for the controller stage; PA1 (f) minimum offset error; and PA1 (e) minimum power consumption. PA1 2. Reduce G.sub.ideal. However, this parameter is dictated by how small the input signal is and how big an output amplitude is desired. PA1 3. Reduce the common-mode voltage. However, this is mainly decided by the structure and operation of the system. PA1 4. Select precision amplifier to reduce V.sub.OS and I.sub.OS. However, this will normally increases the cost significantly.
Unfortunately, most sensors cannot satisfy all of these requirements, and most sensors (e.g., thermal couples) inherently output very low amplitude signals. In addition, due to the existence of wideband, common-mode noise in the harsh automotive environment, the sensor signals are distorted. To deal with these situations, sensors with inherent electrical isolation (e.g., hall effect sensors for electric current measurement) or extra isolation amplifier stages are inserted to reject common-mode noise. However, high cost and wide operating temperature range, cause these techniques to often be unacceptable for automotive applications.
To measure a floating signal with wide bandwidth, a few well-known techniques have been used in the past. For example, if the common-mode voltage is within a few volts and the desired signal is large, a high quality differential amplifier stage, such as shown in FIG. 1 has been used that includes a high performance operational amplifier 10 and a networks of matched resistor 12-22. The ideal output of this circuit can be expressed by the following equation: EQU Assume R.sub.12 =R.sub.14, R.sub.16 =R.sub.18, and R.sub.20 =R.sub.22,
then EQU (V.sub.out).sub.ideal =G.sub.ideal V.sub.sig ( 1)
where ##EQU1## In FIG. 1, V.sub.bias is an optional bias source to shift the amplifier input voltages to desired ranges, and R.sub.20, R.sub.22 form an optional resistor network for attenuating the common-mode input voltage. For example, if the common-mode voltage V.sub.cm is hundreds of Volts, attenuation is necessary to reduce the amplifier terminal voltages to the typical .+-.10 Volts range.
If the resistors and the operational amplifier are not perfect, the actual output voltage in the worst case can be approximated by: ##EQU2## where ERROR=4 (%R) (G.sub.ideal V.sub.cm +G.sub.2 V.sub.bias)+G.sub.3 (V.sub.OS +I.sub.OS R.sub.eq);
(%R) is the tolerance of resistors;
V.sub.OS is the offset voltage of the operational amplifier;
I.sub.OS is the offset current of the operational amplifier;
R.sub.eq =R.sub.12 //R.sub.14 //R.sub.20 =equivalent parallel resistance of these three resistors,
and ##EQU3## This equation also shows how the output error may be reduced: 1. Use precision, matched resistors (or resistor networks). However, resistors with tolerance better than 1% are expensive. An alternative way is to sort out 1% resistors to get matched resistor pairs to approach 0.2% or even o.1% accuracy. Better accuracy beyond this is difficult without excessive cost penalty.
The following typical numbers will provide a better understanding of which parameter(s) dominate(s) the output error:
(%R)=0.1% (0.001)
V.sub.OS =2 mV
I.sub.OS =100 nA
R.sub.eq =10 K.OMEGA.
and assume
V.sub.cm =10 V, V.sub.bias =0 V
and
G.sub.ideal =10, G.sub.2 =0, G.sub.3 =11
then, the error output voltage is:
______________________________________ ERROR = 4 (%R) (G.sub.ideal V.sub.com + G.sub.2 V.sub.bias) + G.sub.3 (V.sub.os +I.sub.ox R.sub.eq) = 0.004 (10*10V + 0) + 11 (2mV + 100nA*10K), = 0.004*100V + 11 * 3mV = 0.43V ______________________________________
This result shows that the common mode voltage and the tolerance of resistors are the major error contributors. If the common-mode voltage cannot be reduced, the only choice is to increase the accuracy of resistors. Unfortunately, this becomes impractical when the common-mode voltage is too high or the error tolerance is too tight. Under this situation, isolation amplifiers are normally chosen to achieve good performance, and various schemes are currently available for this purpose in the market. They are transformer-coupled isolation amplifiers, Sigma-delta modulation opto iso-amps and capacitively coupled iso-amps.
Transformer-coupled isolation amplifiers provide very good galvanic isolation, very high signal gain and reasonable offset, but the frequency bandwidth is limited to a few Khz, and the cost is high compared to other products.
One of the main limitations of the opto isoamps is the operating temperature range. The other limitation is the output common-mode voltage range which will be further amplified if the output requires amplification to meet the controller requirements. This would require calibration of every unit and would be expensive. Also, this approach requires 5 V power supplies and regulators since the automotive battery voltage is typically 12 volts. All of these requirements increase cost. Opto iso-amps also have built in propagation delays that may present a problem for very high frequency operation.
One of the limitations of the capacitively coupled isolation amplifiers is the ripple voltage at the carrier frequency that is amplified by the post amplifier circuits. Also, capacitively coupled systems do not have high input to output common mode transient rejection.
Some sensors are inherently isolated, but have limitations that prevent high performance or operation in harsh applications. For example, both current transformers (CTs) and hall-effect current sensors have no conductive connection to the measured circuit. However, the CTs cannot measure dc currents, and the hall sensors are normally bulky, cannot work in the extended automotive temperature range, and are not very effective in small current measurement. Also, both sensors introduce additional stray inductance to the measured circuits.